The invention relates to a new process for the synthesis of metallocene compounds and the metallocene compounds having a racemic structure, which are useful as a polymerization catalyst for olefins.
Metallocene compounds, in which a cyclopentadienyl, indenyl or fluorenyl groups, or a derivative thereof are made a ligand, are useful as a polymerization catalyst for olefins such as ethylene, propylene or the like under the coexistence of a cocatalyst, for example, aluminoxane. For the preparation for a stereoregular polyolefine have been examined metallocene compounds having various kinds of stereostructures. For a syndiotactic polyolefine preparation is effective a metallocene compound having the Cs symmetry (J. Am. Chem. Soc., 110, 6255 (1988), whereas it is reported that for an isotactic polyolefine preparation is effective a metallocene compound having a racemic structure (Angew. Chem. Int. Ed. Engl., 24, 507 (1985); J. Am. Chem. Soc., 109, 6544 (1987); Chem. Rev., 92, 965 (1992); Organometallics, 13, 954 (1994); Organometallics, 13, 964 (1994)).
Conventionally, synthesis of a metallocene compound having a racemic structure is shown in Reaction Scheme I described below, and it is carried out by reacting the dianion produced by the deprotonation of the ligand and metal tetrachloride or its tetrahydrofuran adduct. ##STR2##
However, since this method gives tarry substances as byproducts, the procedure to separate the aimed metallocene compound having the racemic structure is very complicated (Angew. Chem. Int. Ed. Engl., 24, 507 (1985); J. Organomet. Chem., 288, 63 (1985); Japan Chemical Society ed. Organometallic complex (4th ed. Experimental chemistry series No. 18), Maruzen (1991) p. 81 (in Japanese)), and in many cases a metallocene compound having a meso structure is produced as a byproduct in nearly same amount, whereby there is such a problem that the separation of the aimed racemic metallocene compound is tedious. Generally, since metallocene compounds having the meso structure decrease the efficiency as a stereoregular polymerization catalyst, they are removed by a combination of purifying operations such as column chromatographic, washing or recrystallizing methods (J. Organomet. Chem., 232, 233 (1982); J. Organomet. Chem., 369, 359 (1989); Chem. Lett., 1853 (1989), Organometallics, 10, 1501 (1991); J. Organomet. Chem., 415, 75 (1991); Organometalllics, 13, 954 (1994); J. Organomet. Chem., 497, 43 (1995).
Thus, the conventional synthetic methods give a considerable amount of meso metallocene compounds as byproducts, therefore, the yields of racemic metallocene compounds are low, and due to the fact that the procedure in the purification step is tedious, the cost of the synthesis is high, and it is difficult to carry out in an industrial scale.
As a trial for solving the above problems is reported the method that after the reaction only the metallocene compounds having the racemic structure are crystallized appropriately by selecting the reaction solvent (JP, A, 6-122692; U.S. Pat. No. 5,391,790; U.S. Pat. No. 5,616,747), though, in this method the metallocene compound having essentially meso structure is produced in a nearly half amount, therefore, it can hardly be said that this is an efficient synthetic method.
Further, although the methods by which the metallocene compounds having the meso structure are converted to the metallocene compounds having the racemic structure are studied (J. Organomet. Chem., 342, 21 (1988); Organometallics, 11, 1869 (1992)), a compound of the enough purity could not be obtained, and furthermore, the decomposition of the metallocene compounds occurred.
On the other hand, the method to selectively synthesize the metallocene compounds having the racemic structure is also reported. In the methods using the ligand in which the bulky substituent is introduced to a cyclopentadienyl skeleton and the ligand having a binaphtyl skeleton in a bridging part, they give low yield and are impractical (Organometallics, 10, 2349 (1991); Organometallics, 10, 2998 (1991)). The method to carry out the reaction at the low reaction temperature of -78.degree. C. is also proposed (JP, A, 1-197490; U.S. Pat. No. 5,103,030), but the yield expected is not obtained. Further, in the synthetic examples of the metallocene compounds having the pseudo-racemic structure are used the ligand having the special structure in which 2-position of the indenyl group is bridged by the biaryl group, and the method is practically poor (Organometallics, 12, 2879 (1993); Organometallics, 12, 4391 (1993)). Also, a method employing (CH.sub.3).sub.2 ZrCl.sub.2 has been studied, but the selectivity of the racemic structure amounts at most around 75% in this method. Moreover this might cause trouble in that the reaction tends to take place to predominantly form a meso-type depending on the structure of the ligand (J. Organomet. Chem., 535, 29 (1997)).
On the contrary are reported the methods to obtain the metallocene compounds having the racemic structure in high yield by reacting Zr(NMe.sub.2).sub.4 with the ligand (WO 95/32979; U.S. Pat. No. 5,495,035; Organometallics, 14, 5 (1995); J. Am. Chem. Soc., 118, 8024 (1996)). However, these methods require such a long time as 3-24 hours at the reaction temperature of 80.degree.-160.degree. C., usually 100.degree. C. In the reaction condition, in which heating is done for long time like this, there is a possibility of the polymerization or the decomposition of a ligand which is unstable for heat, and due to the fact that dimethylamine, the starting material of Zr(NMe.sub.2).sub.4, is a gas (b.p. 7.degree. C.) at room temperature, a special reaction equipment corresponding to the gas reaction to prepare Zr(NMe.sub.2).sub.4 is necessary. Further, on the treatment there are various kinds of problems such that Zr(NMe.sub.2).sub.4 is very unstable for air.